The present invention relates to a means retaining intact the surface charge density on electretized air-filter media for a longer period of time and in high density conditions.
A method for manufacturing a filter medium for air-filters wherein porous sheet material comprising synthetic fiber or the like is electretized is disclosed in Japanese Patent Application Publication No. Sho-51-115234, entitled "Method for electretizing porous sheet material", the applicant being the present applicant. Another disclosure of this sort by the same applicant is in Japanese Patent Application Publication No. Sho-51-116837, entitled "Method for manufacturing a filter medium for air-filters". The electret according to said two disclosures adsorbs fine particles suspended in the air by the coulomb force of the positive and negative charges on its two sides. At the same time, it must meet the same conditions that are required of ordinary air-filter media, such as suitable mesh fineness and low susceptibility to air resistance.